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Nanoshocks in materials |
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Dana D. Dlott (Curriculum Vitae) Dana D. Dlott (personal home page) David M. Dlott (personal home page) |
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In our early nanoshock work, we concentrated on developing and perfecting the technique. The apparatus diagrammed at right was developed. It uses a Nd:YLF laser to generate shocks at a high repetition rate, and a pair of synchronously pumped dye lasers to generate a CARS spectrum of the sample. CARS is an acronym for "Coherent Anti-Stokes Raman Spectroscopy". CARS is used instead of ordinary incoherent Raman scattering because the coherent CARS signal is much more intense, which is needed because the sample is very small (1 ng) and the experimental conditions surrounding shock wave generation are very severe. One difficulty with nanoshocks is that the velocity of shock wave propagation (~3 km/s) is much slower than the velocity of the optical pulses (~3 x 105 km/s). To account for this time-of-flight mismatch a very thin sample, usually a few hundred molecules thick, must be used. Another difficult is every nanoshock destroys the shocked sample volume, so fresh sample must be used every time. To solve these difficulties, we use the microfabricated shock target array system shown at right. Our initial experiments used anthracene crystals. Anthracene is regarded as a model system for molecular solids. CARS spectra of a 700-molecule thick anthracene layer before shock and just after the shock hits the anthracene are shown at right. The spectrum shows characteristic frequency shifts and broadening, which can be used to determine the temperature and pressure. An important application for nanoshocks is to understand shock-induced chemical reactions. A model system for shock chemistry is the loss of N2 from an azide, M-N3. At right are some initial results obtained on a thin azide layer. The vibrational transition clearly visible before shock is the symmetric azide stretch. After the shock hits, this peak dimishes in intensity and a broad peak appears in the region expected for condensed phase nitrogen. We have also used nanoshocks to study fast structural relaxation dynamics in polymers and proteins. We have developed a unique energy landscape view of shock compression that is being used to understand biomolecular dynamics and aging of materials. In more recent work, we have begun to look at shock compression of materials with nanostructure. In our first study, we looked at the collapse of nanometer sized voids in porous materials. Void collapse is important in the initiation of energetic materials. A viscoplastic compression model was successfully used to describe picosecond void collapse. We plan to use nanofabrication techniques to produce new engineered nanostructures to better understand shock compression at the microscopic level. To view a presentation on nanoshocks in materials, click here. To view a presentation on nanoshock compression of porous materials, click here.
REPRESENTATIVE PUBLICATIONS Nanoshocks in
molecular materials, Dana D. Dlott, Acc. Chem. Res. 33, pp. 37-45 (2000). Ultrafast
dynamics of shock waves in polymers and proteins: the energy landscape, Hackjin Kim,
Selezion A. Hambir and Dana D. Dlott, Phys. Rev. Lett. 83, pp.
5034-5037 (1999). Shock compression of organic polymers and proteins: ultrafast structural relaxation dynamics and energy landscapes, Hackjin Kim, Selezion A. Hambir and Dana D. Dlott, J. Phys. Chem. A 104, p. 4239-4252 (2000). |
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Schematic of nanoshock apparatus with time- resolved CARS probing
Nanoshock generation and measurement with a micro- fabricated shock target array
Coherent Raman spectra of a 700-molecule thick layer of polycrystalline anthracene before and during a nanoshock
Closeup of the 1400 cm-1 anthracene transition while the shock front is in the anthracene layer. The sharp shock front divides the layer into two parts, so two transitions are seen
Ultrafast shock-induced chemical reaction in a simple explosive, C3N12
Ultrafast shock-induced orientation of a polycrys- talline thin film of the high explosive NTO
Energy landscpe picture of shock compression by a short pulse. A nano- shock can be used to produce large-amplitude structural deformation of amorphous materials.
Schematic of shock com- pression of a porous target.
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